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std::is_empty

STD：：是[医]空空

Defined in header ​​
template< class T > struct is_empty;​(since C++11)

如果T是一个空类型%28，也就是说，除了大小为0的位字段之外，没有任何非静态数据成员的非联合类类型，没有虚拟函数，没有虚拟基类，也没有非空基类%29，提供了成员常量。value平等true.对于任何其他类型，value是false...

如果T是非工会类类型，T应该是一个完整的类型；否则，该行为是未定义的。

模板参数

T-a type to check

辅助变量模板

template< class T > inline constexpr bool is_empty_v = is_empty::value;​(since C++17)

继承自STD：积分[医]常量

成员常数

value statictrue if T is an empty class type , false otherwise (public static member constant)

成员函数

operator boolconverts the object to bool, returns value (public member function)
operator() (C++14)returns value (public member function)

成员类型

TypeDefinition
value_typebool
typestd::integral_constant<bool, value>

注记

从空基类继承通常不会增加类的大小，原因是空基优化...

std::is_empty<T>所有其他类型的特征都是空类。

例

二次

#include <iostream>
#include <type_traits>
 
struct A {};
 
struct B {
    int m;
};
 
struct C {
    virtual ~C(
};
 
int main()
{
    std::cout << std::boolalpha;
    std::cout << std::is_empty<A>::value << '\n';
    std::cout << std::is_empty<B>::value << '\n';
    std::cout << std::is_empty<C>::value << '\n';
}

二次

产出：

二次

true
false
false

二次

另见

is_class (C++11)checks if a type is a non-union class type (class template)

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在CreativeCommonsAttribution下授权-ShareAlike未移植许可v3.0。

http：//en.cppreference.com/w/cpp/type/is[医]空空

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